The present invention relates to a tool monitoring system for monitoring the condition of an electric motor driven tool performing a cyclical operation.
Tool condition monitoring is one of the major concerns in modern machining operations, especially in machining operations for mass production. Failure to detect tool failure and wear leads to poor product quality and can even damage machine tools. On the other hand, a false detection of tool failure or wear may cause an unnecessary interruption of an entire production. Both can result in significant monetary loss.
Known tool monitoring systems include systems for "on-line tool condition monitoring." In on-line tool condition monitoring, the tool is monitored for defects after each cut or cycle. These tool monitoring systems typically use optical sensors or laser optical sensors which measure the geometry of the tool after each cut. However, on-line tool condition monitoring can only detect catastrophic failure of a tool after a cut and cannot monitor the gradual wear of a tool or predict the tool's failure. Further, these systems are vulnerable to chips, coolant, and environmental noises.
Other known methods for tool condition monitoring attempt to predict tool condition based on various sensor signals such as cutting force, acoustic emission, and vibration. However, sensors for monitoring cutting force are too expensive to use with multiple stations and multiple spindles. Acoustic emission and vibration sensors require additional wiring and are vulnerable to various noises.
Some monitoring systems monitor power consumption (or motor current) of the tool. As the tool wears (or if it fails) its power consumption changes. However, the power signals are complicated and the power signals to provide a reliable, accurate indication of it has proven difficult to use. The power signal does contain some "noise" due to factors other than tool condition. Typically, these systems set a range of signal that a monitored signal should fall within. When the monitored signal is outside this range, a worn tool or failure is indicated.
One major problem with monitoring the power consumption of the motor is that occasional spikes are experienced in a machine tool even under normal condition. The spikes can falsely indicate that the tool is worn. However, if the threshold is increased to prevent false signals, a worn tool may go undetected.
The inventors of the present invention previously developed a tool monitoring system which operates generally in two modes: learning mode and monitoring mode. In learning mode, the tool monitoring system gathers statistical data on the power consumption of tools of the selected tool type during learning cycles. A power threshold is generated based upon the statistical data. The tool monitoring system then counts the number of crossings by each of the learning cycles of the power threshold and generates statistical data regarding the number and value of crossings. Preferably, the mathematical operation of wavelet packet transform is used to calculate the power threshold. Feature wavelet packets of the power consumption signal of the tool are calculated. The power consumption signal is then reconstructed from the feature wavelet packets and used to determine the power threshold. In monitor mode, the tool monitoring system counts the number and value of crossings of the power threshold by the power consumption signal of a tool in operation. The tool monitoring system identifies a worn tool, or a tool breakage, or that an abnormal fault happened in the operation, when the number of crossings increases to certain numbers or a certain value relative to the crossings by the learning cycles. This previous invention was disclosed and claimed in U.S. Pat. No. 5,587,931 and application Ser. No. 08/901,609.